Question

i don’t understand why we use getLayeredPane() before setting the contentPane, in this example :

private static void createAndShowGUI() {
    //Create and set up the window.
    JFrame frame = new JFrame("RootLayeredPaneDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Create and set up the content pane.
    RootLayeredPaneDemo newContentPane = new RootLayeredPaneDemo(
      frame.getLayeredPane());
    newContentPane.setOpaque(true); //content panes must be opaque
    frame.setContentPane(newContentPane);

    //Display the window.
    frame.setSize(new Dimension(300, 350));
    frame.setVisible(true);
}

and then in the constructor we just re-use it (layeredPane) :

public RootLayeredPaneDemo(JLayeredPane layeredPane)    {
        super(new GridLayout(1,1));

        //Create and set up the layered pane.
        this.layeredPane = layeredPane;

But in another example, we just forget the parameter, for example (new LayeredPaneDemo()):

private static void createAndShowGUI() {
    //Create and set up the window.
    JFrame frame = new JFrame("LayeredPaneDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Create and set up the content pane.
    JComponent newContentPane = new LayeredPaneDemo();
    newContentPane.setOpaque(true); //content panes must be opaque
    frame.setContentPane(newContentPane);

    //Display the window.
    frame.pack();
    frame.setVisible(true);
}

and the constructor :

public LayeredPaneDemo()    {
    setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));

    //Create and set up the layered pane.
    layeredPane = new JLayeredPane();

?

Answer 1

Ok so the first example refer to http://docs.oracle.com/javase/tutorial/uiswing/components/rootpane.html#layeredpane
and the seconde one refer to : http://docs.oracle.com/javase/tutorial/uiswing/components/layeredpane.html

and the answer is told in the first ref. :

(This) is a version of LayeredPaneDemo that uses the root pane’s layered pane, rather than creating a new layered pane

RootPane are used to manage layers using their references instead of creating new layer… quite clear I think.